Method and bath for electroplating gold



United States Patent 3,397,127 METHOD AND BATH FOR ELECTRO- PLATING GOLD Eldridge K. Camp, Watertown, Conn., assignor to American Chemical & Refining Company, Incorporated, Waterbury, Conn., a corporation of Connecticut No Drawing. Filed Apr. 12, 1965, Ser. No. 447,536 13 Claims. (Cl. 204-46) ABSTRACT OF THE DISCLOSURE There is disclosed a method of plating gold wherein a soluble gold cyanide complex salt is dissolved in water and the resultant solution acidified to a pH of about 2.0- 5.0 to expel substantially all free cyanide therefrom. The pH of the solution is thereafter raised to a value of about 6.110.5 at which the plating operation is conducted with an electroplating current to produce gold plated articles having uniform surfaces with a low edge to center ratio.

The present invention relates to a novel bath and method for plating high-purity gold.

Generally, the more commonly employed baths for the electroplating of gold utilize cyanide ion in excess of the amount of gold present. Because of the noxious nature of hydrogen cyanide, such baths customarily are employed at a highly alkaline pH with limitations as to application and hazards to personnel. There have been proposals for cyanide-free baths set forth in Smith United States Patent 3,057,789 and Ehrhardt French Patent 1,265,517. These patents and others have proposed baths which might be operated at acid or nearly neutral pH including Volk United States Patent 2,812,299. However, one of the most common shortcomings of various baths for electroplating gold has been a tendency for a much greater deposit thickness adjacent the edges of the workpiece than at the center with resultant increased cost to achieve the minimum thickness.

It is an object of the present invention to provide a novel bath and method for the plating of gold which will yield a low edge to center ratio in the thickness of the plated metal with readily controllable deposit thickness.

It is also an object of the present invention to provide such a bath and method having extremely high covering power and relatively long life and which is relatively economical in operation.

Another object is to provide such a bath and method which yield an ultrahigh purity deposit of gold having high ductility and a desirable controlled appearance.

Still another object is to provide a plated article having a deposit of gold thereon with a low edge to center ratio and desirable controlled appearance and high ductility.

It has now been found that the foregoing and related objects can be readily attained by a method in which a soluble gold cyanide complex salt is dissolved in water to provide a solution which is then acidified to a pH of about 2.0-5.0 to drive olf some cyanide therefrom. The pH of the solution is then raised to about 6.110.5 for the plating operation which generally will be electroplating although the bath may also be employed for chemical plating.

By use of the present invention it has been found possible to consistently obtain gold deposits of extremely high purity having an edge to center ratio of less than 1.421 and generally less than 13:1 and even below 1.2:1. The deposits have a desirable light metallic yellow appearance and a reflection which may vary from lustrous to satin An unsupported electroplate has been bent upon itself without evidence of cracking and the deposit has been 3,397,127 Patented Aug. 13, 1968 found to have a Vickers hardness within the range of 50.0 to 75.0 kilograms per square millimeter at a 50 gram load. The bath may be operated at a wide range of current density varying from 0l6.5 amperes per square decimeter with a relatively high degree of current efiiciency.

The theory of operation of the present invention is not fully understood. Many commercial gold cyanide complex salts contain cyanide ion in excess of two mols of cyanide to every mol of gold. Even with those salts which are of high purity so as to have the proper molar equivalents, the present invention has been found applicable in that some portion of the combined cyanide appears to be eliminated in the form of hydrogen cyanide while still retaining some amount of the gold as the soluble gold cyanide complex salt. In subsequent operation of the bath even at temperatures at or near boiling, the fact that there is minimal evolution of hydrogen cyanide and very little build up of carbonates in the bath is quite significant. It is believed that the cyanide retained in the acidified and subsequently neutralized bath is suflicient to produce enhanced operation while at the same time avoiding the development of any excess cyanide ion such as might produce deleterious elfects.

The soluble gold cyanide complex salts employed in accordance with the present invention should not introduce any interfering cations. Generally the salts which fall within this category may be classified as alkali metal and ammonium gold cyanide. Potassium gold cyanide and sodium gold cyanide have proven particularly effective and are readily available. The gold content may range from 4.0 to 500.0 grams per liter and preferably 6.0 to 15.0 grams per liter.

The solution of the complex salts may be acidified to a pH of 5.0 or below by use of an acid or acid salt which will not introduce an interfering anion or cation. Phosphoric acid, sulfuric acid, sulfamic acid, hydrochloric acid, tartaric acid, acetic acid, nitric acid and citric acid or the acid salts thereof, have all been employed with varying degrees of effectiveness. Phosphoric acid and its acid salts have been found preferable for operation in the present invention although acetic acid and tartaric acid also have produced desirable results.

Although the expelling of the cyanide ion will occur slowly at ambient temperatures particularly at a low pH, generally it is desirable to accelerate the procedure by use of elevated temperatures and agitation. For example, at a pH of approximately 4.0, the acidified solution may be merely brought to a boil rapidly and cooled or it may be held at a temperature of approximately 77 C. for a period of up to one hour. The conditions employed should not destroy all the gold cyanide complex salt present in the solution.

After the expulsion of the cyanide ion, the pH of the bath is then elevated to the range of 6.ll0.5 and preferably about 6.3-7.3 for optimum operation for most applications. A non-interfering base such as ammonium or the alkali metal hydroxides may be employed for this purpose or suitable basic salts may also be used such as diamrnonium phosphate, disodium phosphate, etc.

The agents used to acidify and neutralize the solution should provide a current-carrying radical or anion in the amount of at least 10 grams per liter and preferably 15 grams per liter, and up to saturation. Although various anions have been tested and utilized with varying degrees of effectiveness as set forth herebefore, the most desirable operation has been obtained by use of phosphates as the acidifying and neutralizing agents, although a highly desirable degree of operation has been observed with respect to tartrates and acetates. The data concerning the efiicacy of the several anions is set forth in Table 1 below.

TABLE I [53-78 microinch gold deposits] Current Time, Deposit C.C.E., Anion Typo Density, Minutes Thickness, mg./a.m. p11

mat/cm. 2 nag/cm.

Phosphate 3.1 10 3. 5 117 6. 3 Sullamate-Sulfate..- 3. 1 10 3. 2 105 0. 3

2-49 microinch gold deposits] Phosphate 3. 9 5 2. 4 124 6. 3 Sullamate-Sullate-.. 3. 9 5 2. l 108 6. 3 Acetate 3.9 5 2. 1 108 6. 3 Chloride... 3. 9 5 2. 104 6. 4 Nitrate. 3. 9 1. 6 80 12 Tartrate... 3. 9 5 2. 2 112 6. 3 Citrate 3. 9 5 1. 7 8S 6. 1

[380-590 microinch gold deposits] Phosphate 9 60 27 116 0. 3 Sulfamato-Sul 3. 9 60 Acctate.... 3. 4 6O 29 123 6. 3 Chloride... 3.9 60 26 111 6. 4 Nitrate 3. 9 60 19 83 12 Tartrate 3. 9 (i0 28 120 6.3 Citrate 3. 9 60 25 106 6. 1

As a specific example of a preferred embodiment of the present invention, grams per liter of potassium gold cyanide are dissolved in water and 100 grams per liter of ammonium dihydrogen phosphate are added thereto to lower the pH to about 4.5. The resultant solution is held at 77 C. for one hour with stirring to expel cyanide. Thereafter 15 grams per liter of sodium hydroxide are added thereto and the pH is adjusted to 6.3 with ammonium hydroxide. This bath may be operated at 65 77 C. With a current density of 0.550.77 ampere per square decimeter for rack plating and 0.11-0.33 ampere per square decimeter for barrel plating.

The baths of the present invention may be operated at a Wide range of current density varying from 0.0 to 16.5 amperes per square decimeter with varying degrees of current efiiciency. Generally, the bath is most efiicient by operation at current densities of 0.0 to 1.5 amperes per square decimeter. For rack operation, the bath is most desirably operated at 0.3 to 0.8 ampere per square decimeter; and for barrel operation, the bath is most desirably operated at 0.1 to 0.4 ampere per square decimeter.

The bath may be operated at a temperature range of 38 C. to boiling depending upon the particular process and application. For immersion or chemical plating wherein no current is employed, the temperature may be as low as 38 C. for plating upon copper and is preferably above 57 C. for plating upon nickel, iron and molybdenum. For electroplating operations to achieve a good appearance, the temperature should be above 65.5 C. and the bath may be operated efficiently even at boiling although the preferred range is 65.5-75.5" C.

Various anodes may be employed including gold, stainless steel, platinum, platinum-clad tantalum and graphite. In addition, a stainless steel tank may be used as the anode. In accordance with usual practice, the tank should be one which will be resistant to attack by the bath. The bath should be filtered and agitated to avoid difficulties and to obtain optimum operation.

Exemplary of the efiicacy of the present invention are the following specific examples:

.4 z Examplel A bath Was prepared by dissolving in water 15 grams per liter of potassium gold cyanide and acidifying the solution with grams per liter of ammonium dihydrogen phosphate. The bath was heated to a temperature of 77 C. for a period of about one hour with agitation to expel cyanide ion therefrom. Thereafter'the solution was neutralized to a pH of approximately 6.3 with ammonium hydroxide.

Two and one-quarter gallons of the bath were placed in a stainless steel tank and the temperature of the bath was maintained at about 65.5-72.5 C. with agitation and filtration. A nickel sheet of 0.0063 inch thickness, 4.5 inches long and 0.873 inch in length was first subjected to alkaline cleaner, rinsing, mild acid treatment and rinsing and then placed in a rack. The tank was employed as the anode and the current was 0.25-0.35 ampere except for a short period following immersion when the current was maintained at 0.10 ampere.

Upon removal from the bath, the cathode was rinsed and dried. The resultant plate was found to have a pleasing light yellow metallic appearance and the surface Was relatively rough like a wrinkle finish. The thickness of the deposit was found to follow the burr of the metal sheet used as the cathode. Upon measurement, the thickness at various points was as follows.

TABLE 2 Location: Thickness, inches One inch from bottom at center 0.096 One inch from bottom at first side 0.1240 One inch from bottom at second side 0.1220 One and one-half inch from top at center 0.096

One and one-half inch from top at first side-.. 0.1171 One and one-half inch from top at second side 0.1175

Thus, in computing the edge to center ratio derived at one inch from the bottom, the result is 1.29 at 0.04 inch of deposit. At one and one-half inches from the top, the edge to center ratio is 1.22. The corner to center ratio which could be the highest possible for this piece taken from the dead center to any of the four corners is 0.147/0.095 or 1.55. The edge to center ratio taken on the Width dimension was found to be 1.19/0.997 or less than 1.20. Because the edges of the sheet metal cathode had such burrs that the thickness was 0.0073 inch on one side and 0.0068 inch on the other side with an edge to center ratio for the unplated metal of 1.1 to 1.16 and because electroplating with gold tends to exaggerate defects in the substrate, the above determined edge to center ratios were regarded as exceptionally good.

Example 2 In a bath of the composition of Example 1, an electroform Was prepared having a thickness of 25 mils. This electroform was subjected while unsupported to a temperature of 899 C. for three hours without blistering, discoloration or distortion.

Example 3 i A series of baths were prepared by dissolving 16.67 grams per liter of potassium gold cyanide in water and acidifying the solution to a pH below 4.5 with an acid or acid salt. After heating to drive off some of the TABLE 3 Compound. 1 2 3 4 5 6 7 Sulfamic acid, g 7. 2 Potassium sulfate, g Ammonium hydroxide, Acetic acid, ml

Sodium hydroxide, g..-.. Hydrochloric acid, m1

Sodium chloride, g Tartaric acid, g Potassium nitrate, g. Nitric acid, ml Potassium hydroxide, Citric acid, g Ammonium dihydrogen phosphate,

cyanide, the pH was adjusted to above 6.1 with ammonium hydroxide alone in Baths 1, 4, 6 and 7; by sodium hydroxide and ammonium hydroxide in Baths 2 and 3; and by potassium hydroxide in Bath 5. Additional salts were added in some instances to provide sufficient current-carrying anion. The several substances added to the solution of potassium gold cyanide are set forth in Table 3 above. Test panels were prepared by plating at a temperature of about 67 C., the current density, time, deposit thickness, and pH of the several baths being shown in Table 1 hereinbefore for three separate test panels prepared in each bath. The test panels were nickel panels one-half inch by two inches and five mils in thickness. The baths were operated at a temperature of about 69 C.

In all instances, a satisfactory gold deposit was obtained but the color of the panels produced by the sulfamate and chloride baths was a little orange-like. As can be seen, the nitrate and citrate baths were low in current efficiency. a p 4 A bath was prepared by dissolving ten grams per liter of water of potassium gold cyanide and acidifying the solution with 50 grams per liter of ammonium dihydrogen phosphate. After heating the bath to expel cyanide therefrom, the bath was neutralized to a pH of about 6.5 by addition of 5 grams per liter of sodium hydroxide and adjustment to the end point with ammonium hydroxide.

A nickel panel two inches by one-half inch and 5 mils in thickness was immersed therein at a temperature of 77 C. for a period of six minutes with no current applied to the bath. Upon removal the panel was observed to have a bright gold deposit of about three microinches in thickness.

Example 5 Potassium gold cyanide was dissolved in water to provide 400 grams per liter and the solution was then acidified by the addition thereto of 48 grams per liter of ammonium dihydrogen phosphate. After heating to drive off cyanide, 84 grams per liter of diammonium phosphate was added thereto and the pH was adjusted to 6.3 with ammonium hydroxide.

A graphite cathode was immersed therein and the bath was operated at a temperature of 71 C. A current of 4.40 amperes per square decimeter was applied between the anode and cathode. The resultant metallic deposit had a satin, highly pleasing appearance.

Thus it can be seen from the foregoing detailed specification and example that the present invention provides a novel bath and method for the plating of gold which yields a low edge to center ratio in the plated metal and facile control of the thickness of the deposit. The bath is relatively economical in operation and has extremely high covering power and long life. The gold deposit produced thereby has high ductility, a desirable controlled appearance and a satin finish particularly in the heavier deposits which are possible herewith. Electroforms produced therefrom can tolerate temperatures approaching the melting point without blistering, distortion or discoloration.

Having thus described the invention, I claim:

1. In the method of electroplating gold, the steps comprising dissolving in water a complex gold salt selected from the group consisting of ammonium gold cyanide and alkali metal gold cyanide in an amount sufficient to provide 4.0 to 500 grams per liter of gold; acidifying the resultant solution with an acid agent to a pH of about 2.0 5.0; heating the acidified solution to drive off substantially all free cyanide therefrom; and thereafter adding a neutralizing agent to raise the pH of the solution to about 6.1 to 10.5, the acidifying and neutralizing agents providing a current-carrying radical in the amount of about grams per liter to saturation.

2. The method of claim 1 wherein said acidifying agent is a phosphate compound.

3. The method of claim 1 wherein the pH of the neutralized solution is about 6.3 to 7.3.

4. In the method of electroplating gold, the steps comprising dissolving in water a soluble gold cyanide complex salt selected from the group consisting of ammonium gold cyanide and alkali metal gold cyanides in an amount sufficient to provide 4.0 to 500.0 grams per liter of gold; acidifying the resultant solution to a pH of about 2.0-5.0 and expelling substantially all free cyanide therefrom; thereafter raising the pH .of the resultant solution to about 6.1- 10.5; placing in said solution a cathode and an anode; and passing current between said anode and cathode to plate gold upon said cathode to the desired thickness at the center thereof, the plated gold having an edge to center ratio of less than 1.4: 1.

5. The method in accordance with claim 4 wherein said solution is raised to a pH of 6.3 to 7.3.

6. In the method of electroplating gold, the steps comprising dissolving in water a complex gold salt selected from the group consisting of ammonium gold cyanide and alkali metal gold cyanide in an amount sufiicient to provide 4.0 to 500.0 grams per liter of gold; acidifying the resultant solution with an acid agent to a pH of about 2.0-5 .0; heating the acidified solution to drive off substantially all free cyanide therefrom; thereafter adding a neutralizing agent to raise the pH of the solution to about 6.1 to 10.5, the acidifying and neutralizing agents providing a current-carrying radical in the amount of about 15 grams per liter to saturation; placing in said solution a cathode and an anode; and passing a current of about 0.1-0.8 ampere per square decimeter between said anode and cathode to plate gold upon said cathode to the desired thickness at the center thereof, the plated gold having an edge to center ratio of less than 1.4: 1.

7. The method of claim 6 wherein said acidifying agent is a phosphate compound.

8. The method of claim 6 wherein the pH of the new tralized solution is about 6.3 to 7.3.

9. The method of claim 6 wherein said edge to center ratio is less than 1.25:1.

10. A bath for electroplating gold comprising an aqueous solution containing a gold cyanide complex salt selected from the group consisting of ammonium gold cyanide and alkali metal gold cyanides and a current-carrying radical in the amount of 15 grams per liter to saturation, said solution containing 4.0 to 500.0 grams per liter of gold and having a pH of about 6.1 to 10.5, said solution being substantially free from free cyanide ion and having been acidified initially to a pH of about 2.05.0 and heated to drive off substantially all free cyanide.

11. The bath .of claim 10 wherein said solution has a pH of 6.3 to 7.3.

12. The bath of claim 10 wherein said current-carrying radical is a phosphate ion.

13. A bath for plating gold comprising an aqueous solution containing a gold cyanide complex salt selected from the group consisting of ammonium gold cyanide and alkali metal gold cyanides and phosphate ion in the amount of 15 grams per liter to saturation, said solution containing 4.0 to 500.0 grams per liter of gold having a pH of about 6.3 to 7.3, said solution being substantially free from free cyanide ion and having been acidified initially to a pH of about 2.0 to 5.0 and heated to expel substantially all free cyanide therefrom.

References Cited UNITED STATES PATENTS 2,905,601 9/1959 Rinker et a1 204-46 XR 2,967,135 1/1961 Ostrow et al 204-46 XR 2,978,390 4/1961 Atwater et al 204-46 3,123,484 3/1964 Pokras et al 1061 3,303,111 2/1967 Peach 20438 JOHN H. MACK, Primary Examiner.

G. KAPLAN, Assistant Examiner. 

